This invention relates generally to equipment for retrieving data from documents which have been recorded on individual frames of a reel of microfilm. More particularly, the present invention relates to a technique for accurately counting the frames on a reel of microfilm so that the people using the microfilm reader can quickly and automatically stop the microfilm reader at a particular frame to extract data from the document recorded on that frame.
In the initial recording of documents on microfilm it is common to have each page of the document recorded on a separate frame of the film. Hence each frame of the microfilm corresponds to a single page of the document. Typically the microfilm apparatus places a dark spot, called a marker or "blip," on each frame of the film. Then an index is prepared identifying the document and indicating the frame on which the document is recorded.
Since each reel of microfilm may have several thousand frames, it may be appreciated that when it becomes necessary to look at a particular frame of the microfilm, to study a specific document, it will be desirable to be able to quickly advance the microfilm through the microfilm reader and to stop the microfilm automatically at the frame on which the particular document has been recorded.
The advancing of the microfilm to locate a particular document is called "scanning" or "searching." Devices already exist for automatically advancing the microfilm through a reader and for counting the markers or blips on the frames in an attempt to scan or search and automatically stop at the desired document. It may be appreciated, however, that if the scanning operation is incorrect by even one or two frames, then the person operating the microfilm reader, if unskilled at recognizing and analyzing the specific document, may relay on information from the wrong document, because the operator will not know that the microfilm system has automatically stopped at the wrong document.
In the recording of documents on microfilm, with each page being recorded on an individual frame of the film, the frame blips or markers are placed near the edge of the film to avoid interference or overlap with the text of the recorded document itself. Since most microfilm readers rely on a photoelectric system for detecting the frame markers, the markers are typically black spots on the edge of the film--one spot or marker for each frame.
The photoelectric system usually comprises a light source on one side of the film and a phototransducer on the other side of the film. The transducer is a photo-transistor which generates a current depending upon the amount of light received from the light source and passing through the microfilm. When a frame marker appears in the path of the photoelectric system, the marker interrupts or blocks the light to the photo-transistor and thus the output of the photo-transistor, i.e., the current generated by the photo-transistor, changes to indicate that a marker has moved between the light source and the photo-transistor. This should indicate that another frame of the microfilm has moved through the microfilm reader.
However, if smudges, fingerprints or stray marks appear on the microfilm near the edge of the film, the light to the photo-transistor will be interrupted which also changes the output from the photo-transistor. Hence this would falsely indicate a frame marker.
Thus one drawback of the optical system of the prior art is the false marker problem which can arise because of smudges, marks or fingerprints on the microfilm. This may give rise to false "countings" and distort the entire scanning or search operation.
Another difficultly with the optical systems of the prior art is the insufficient optical "contrast" between the darker frame marker and the lighter remaining portion of the microfilm frame. This may be caused by one or more factors including the decrease in illumination from the light source caused by age, dirt, etc., the type of microfilm, general quality of the microfilm and quality of the original document. This factor is called the "baseline" of the microfilm which is a measure of the actual light passing through the microfilm and received by the photo-transistor. It is clear that if the general light reception becomes diffused or dim, there may be a failure to detect the frame markers since there may be insufficient optical contrast between the marker and the background.
There are, of course, many prior art techniques for counting frame markers in microfilm reader systems. These techniques, however, have neither recognized nor inherently solved the aforementioned problems. Such prior art techniques, in general, take the analog output from the photo-transistor and compare such analog output with an analog representation of the background light. This comparison, in analog form, is typically accomplished in the prior art by a log or radio comparator which basically subtracted the logarithm of the background light value from the logarithm of the marker light value. At low levels of light, the difference between these two values became smaller and would not exceed the threshold necessary to recognize a frame marker, thus frame markers would be erroneously ignored. These low light levels as previously discussed may be caused by a variety of operating-related defects due to age or dirt on the light source or film. Thus the log or ratio method does not reflect real-life conditions of the photoelectric system such as changing light value or quality of film.
Thus the present invention overcomes the problems of the prior art by ignoring stray marks, smudges, and the like and by adjusting for changing conditions of the light source and the film.